If you re-install windows, you will delete everything on your hard drive.
I'm just suggesting something else to try first. When you turn your computer on, it usually checks where to boot FROM. It'll check your floppy, your CD drive or your HDD or all of them, depending on your settings.
It is quite possible that your computer is not looking at your Hard drive. It is ignoring it. Bios has nothing to do with windows. When you boot up your computer, hit f8 every couple of seconds until you see it loading into BIOS.
There is a setting in there that tells your computer where you want it to look when starting your computer. It sounds to me like for some reason, this setting may have been changed. Look for "boot settings" and make sure it is looking at your hard drive.
Others may have some other ideas. My hunch is, even if you re-install windows, you are going to have the same problem.
p.s. why did you switch your ram installation around? Can you give more details on this please. It could also very well be a cause of your headaches.